1. Field of the Invention
This invention relates to an image reading apparatus for computer input, which reads an original such as a photograph or a document, converts it into digital data and outputs the digital data, and an image reading apparatus corresponding to the original reading portion of an analog copier.
2. Related Background Art
The construction of a color image reading apparatus according to the prior art is schematically shown in FIGS. 5A to 5C of the accompanying drawings.
The letter P designates an original to be read, which is placed on an original mounting glass table 100, and a reading unit 101 is scanned in parallel to the original mounting glass table 100 to thereby read an image on the original.
The reading unit 101, as schematically shown in FIG. 6 of the accompanying drawings, has incorporated therein LED's 101R, 101G and 101B of three colors which are light sources for irradiating the original, a rod lens array 101L for imaging the reflected light from the original on the light receiving element of an image sensor, and the image sensor 101S.
The light sources of three colors are successively changed over and turned on and the image sensor 101S reads the reflected light of each color from the original, to thereby effect color resolution reading.
The reading unit 101 is fixedly supported on a slider 102 slidable on a guide shaft 103 fixed to the main body of the apparatus. Also, a belt 104 for transmitting motive power from a motor 105 which is a scanning drive source is fixed on the slider 102.
By the forward and reverse rotations of the motor 105, the reading unit 101 can be reciprocally scanned within the range of the original mounting glass table 100.
The constituents of the image reading apparatus further include an electrical equipment portion 106 comprising a control board and a power source, besides what has been described above.
These constituents are disposed in a housing comprising a combination of an upper cover 112 for fixedly supporting the original mounting glass table 100 and a lower cover 113.
An original cover 111 for pressing the original against the original mounting glass table 100 is openably and closably mounted on the original mounting glass table 100.
FIG. 7 of the accompanying drawings is a read image data processing block diagram by this image reading apparatus. An image output signal read by the image sensor 101 in synchronism with the LED's turned on and off in succession is sent to an amplifier 121 and is amplified thereby, and thereafter is converted into a digital image signal by an A/D converter 122.
The A/D converter 122 divides the dynamic range of the image sensor (the reading output difference between the white portion and the black portion of original) into the bit number thereof, and allots the number of gradations in conformity with the brightness of the image on the original.
For example, when an A/D converter of resolving power of 8 bits is used, white to black can be discriminated into 256 gradation levels, and when an A/D converter of resolving power of 10 bits is used, white to black can be discriminated into 1,024 gradation levels. Accordingly, in an image reading apparatus using the A/D converter of 8 bits, 24 bits=about 16,700,000 colors can be discriminated in the color reading by light sources of three colors R, G and B, and in the case of 10 bits, 30 bits=about 1,074,000,000 colors can be discriminated.
There are several kinds of output forms of the image signal of the image reading apparatus, and depending on the use of an image read, an output form suited therefor can be selected.
When a writing is to be read and the content thereof is to be applied to OCR or when a monochromatic line drawing is to be read, a monochromatic binary image is suitable, and use is made of image data obtained by binarizing an image signal obtained with e.g. only G of the above-described light sources of R, G and B turned on, by a certain threshold value in an image processing circuit incorporated in a gate array 123.
When an image is to be read with a view to read an image such as a photograph and output it to a monochromatic printer, use is made of image data binarized by the use of halftone processing such as a dither method or an error diffusing method using the image signal also by the G light source. When the processing of a color image is to be effected, image data of multiple values (24 bits, etc.) are suitable.
The image signal passed via the image processing circuit is outputted to a computer 200 which is an apparatus such as a personal computer through an interface circuit 124.
In recent years, however, the image reading apparatus according to the prior art as described above has come to be often used in offices and homes with the spread of personal computers. Along therewith, it has been brought to the fore as an important item in the specification of products to make the installation area occupied on a desk and consumed electric power as small as possible.
The pending problem when downsizing the apparatus is that since the span between the bearings of the reading unit 101 in the sub-scanning direction is short, vibration sometimes occurs to the movement of the reading unit 101 due to the fitting backlash between the bearings and the guide shaft 103. An image reading apparatus connected to a computer needs have the function of interruption/resuming the operation thereof in the course reading in conformity with the processing situation of the computer, and for the image before and after the interruption/resumption to be smoothly connected, there must not be such vibration.
Also, a method of making consumed electric power small is to make the electric current supplied to the motor 105 small, and for that purpose, it is necessary to make the driving load of the motor 105 small.